Game animation from other video file into blender animation (adding to the existing video file)
This is possible by assigning to your tablet model "screen" surface (probably a rectangle) a "image or movie" texture (obviously choosing your movie). Of course you need to learn how to correctly assign the texture to your mesh. You could do this either by UV mapping or using a separate material for the "screen" surface...

To add the video after the tablet animation is done, you can use video tracking to map your video to the screen area, but you will need to have  some good tracking feature in the animated screen, to get a good result (you need to be able to set good markers)